1. Technical Field
The present invention relates to a combined sensor provided with a microwave sensor (hereinafter referred to as “MW sensor”)—an active sensor using low-frequency electromagnetic wave rather than visible radiation—and with an infrared sensor (e.g., passive infrared sensor; hereinafter referred to as “PIR sensor”).
2. Prior Art
Combined sensor has been known for use in an intruder detection system (burglar system) as disclosed, for example, by Japanese Unexamined Patent Application Publication No. 11-39574. This type of combined sensor is provided with an MW sensor designed to send microwave toward the detection area, receive the reflected wave from a human body and detect the intruder through Doppler effect and a PIR sensor designed to receive infrared radiation from a human body in the detection area and detect the intruder from the difference between the human body and the ambient temperature. The combined sensor compliments weaknesses of the two sensors and offers improved reliability against false alarms by overlapping the detection areas of the MW and PIR sensors and ANDing their detection counts. False alarm due to external radio wave for the MW sensor and false alarm caused by sunlight for the PIR sensor are among the weaknesses of the two sensors.
This type of combined sensor has the PIR sensor beam range (range in which infrared radiation from the human body is receivable) set on the ground near the outer edge of the detection area, thus discriminating between detection and non-detection areas. For this reason, even if a large-sized object (truck or train) passes by the combined sensor and if the object is detected by the MW sensor (the MR sensor issues an alarm), the combined sensor can be set not to detect the object (issue an alarm) when the object is outside the detection area set by the PIR sensor (outside the beam range).
Incidentally, since the detection area of the aforementioned combined sensor is determined by the PIR sensor beam range, it may be impossible to set the beam position on the ground near the outer edge of the target detection area in the event of a combined sensor mounting error (mounting angle error). Thus, if the beam position is not set on the ground near the outer edge of the target detection area, the detection area of the combined sensor may deviate considerably from the target detection area. FIG. 7 shows a deviation of the detection area (beam position) as a result of mounting error of a sensor 70 mounted on the wall of a house or other location. Suppose, for example, that one wishes to set the area for object detection (target detection area) to a point at a distance of up to 30 m. As long as the mounting angle of the combined sensor is correct, the beam position is set on the ground at the 30 m point (point I in the figure). In contrast, if the sensor is mounted pointing more downward than it would at the correct mounting angle, the area for object detection is shorter than the 30 m distance point (refer to the alternate long and short dashed line in the figure). Conversely, if the sensor is mounted pointing more upward than it would at the correct mounting angle, the area for object detection is longer than the 30 m distance point (refer to the alternate long and two short dashed line in the figure).
Table 1 shown below depicts an example of relationship between mounting angle error of the combined sensor on the wall surface and distance to the outer edge of the detection area (maximum detection distance) as a result of the error. Assuming that the combined sensor mounting height is 3 m, the target detection area is up to the 30 m point and the appropriate mounting angle of the combined sensor is 84.3°, Table 1 shows the case in which ±3% error (with upward error as positive) may result as the mounting angle error of the combined sensor (maximum error). This mounting angle error may not only occur during mounting of the combined sensor but also as a result of change over time.
TABLE 1Angle (deg)Max. detection distance (m)81.3 (−3)19.682.3 (−2)22.283.3 (−1)25.584.3 (0)3085.3 (+1)36.586.3 (+2)46.387.3 (+3)63.6
As is apparent from Table 1, −3° error (pointing downward) in the combined sensor mounting angle leads to a difference of only 10 m or so from the target detection area while +3° error (pointing upward) in the combined sensor mounting angle results in a difference of as much as about 33 m from the target detection area. This means that despite the original intention to perform object detection only up to the 30 m distance point, objects existing in the area up to a distance of about 63 m are subject to detection due to the mounting angle error. In other words, the combined sensor issues a false alarm if there is an object in the area at a distance of up to approximately 63 m—an area beyond the 30 m point. This is the reason why further improvement has been needed to enhance sensor reliability.
The present invention was conceived in light of the foregoing. It is therefore the object of the present invention to provide a combined sensor, provided with MW and infrared sensors, capable of detecting objects only within the desired detection area even in the event of an error in the sensor mounting angle.